power: qpnp-bms: fix divide by zero bug in calibration

Sometimes the iadc may return zero for gain during a calibration
reading. This causes a divide-by-zero bug in the BMS driver.

Fix this by not adjusting for gain when gain is zero.

Change-Id: Ieadb9fc92576f99692158539e06cdf261f4f49b5
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>